All cultivated forms of lettuce belong to the highly polymorphic species Lactuca sativa that is grown for its edible head and leaves. As a crop, lettuces are grown commercially wherever environmental conditions permit the production of an economically viable yield.
Lactuca sativa is in the Cichoreae tribe of the Asteraceae (Compositae) family. Lettuce is related to chicory, sunflower, aster, dandelion, artichoke and chrysanthemum. Sativa is one of about 300 species in the genus Lactuca. 
Lettuce cultivars are susceptible to a number of diseases such as downy mildew, sclerotinia rot, botrytis, powdery mildew, anthracnose, bottom rot, corky root rot, lettuce mosaic virus, big vein, beet western yellows and aster yellows. These diseases result in millions of dollars of lost lettuce crop throughout the world every year.
Of the various diseases that affect lettuce cultivars, downy mildew (Bremia lactucae) is the most highly destructive of lettuce grown at relatively low temperature and high humidity. Downy mildew is caused by an oomycete, Bremia lactucae Regal, which can be one of the following strains: Bl:18, Bl:20, Bl:22, Bl:24, Bl:25, Bl:26 (Van Ettekoven, K. et al., “Identification and denomination of ‘new’ strains of Bremia lactucae.” In: Lebeda, A. and Kristkova, E (eds.), Eucarpia Leafy Vegetables, 1999, Palacky University, Olomouc, Czech Republic, pp. 171-175; Van der Arend et al. “Identification and denomination of “new” races of Bremia lactucae in Europe by IBEB until 2002.” In: Van Hintum, Th et al. (eds.), Eucarpia Leafy Vegetables Conference 2003, Centre for Genetic Resources, Wageningen, The Netherlands, p. 151), Ca-VII, Ca-VIII (Michelmore R. & Ochoa. O. “Breeding Crisphead Lettuce.” In: California Lettuce Research Board, Annual Report 2005-2006, 2006, Salinas, Calif., pp. 55-68), NL341, and NL519. NL341 (IBEB sextet-code EU-A 55-63-13-01) and NL519 (IBEB sextet-code EU-A 27-63-13-01) were selected, because they are widely occurring in Europe since 2005, and because they are breaking the well-known dm17-gene, which was considered to provide broad resistance to lettuce downy mildew (Van der Arend et al. 1999 & 2003; Michelmore & Ochoa, 2006).
Downy mildew causes pale, angular, yellow areas bounded by veins on the upper leaf surfaces. Sporulation occurs on the opposite surface of the leaves. The lesions eventually turn brown, and they may enlarge and coalesce. These symptoms typically occur first on the lower leaves of the lettuce, but under ideal conditions may move into the upper leaves of the head. When the pathogen progresses to this degree, the head cannot be harvested. Less severe damage requires the removal of more leaves than usual, especially when the lettuce reaches its final destination.
Although several known lettuce cultivars exhibit resistance against downy mildew, irrespective of lettuce type, all the lettuce cultivars affected produce a limited number of leaves that generally are of unequal size and diminished quality with respect to color and shape. This is a distinct disadvantage for processing purposes because leaves either need to be sorted based on size or they need to be cut to a smaller, more uniform size. The first option requires additional labor, with not all sizes usable. The second option has the disadvantage that it creates many cut surfaces, which then are subject to wound-induced browning, resulting in a greatly reduced shelf-life.
Although green lettuce plants exist that have a large number of small, more uniform leaves (U.S. Pat. No. 6,320,104; US patent application 20070022496), they all lack resistance to newly appearing lettuce downy mildew strains, like the ones mentioned above: Bl:18, Bl:20, Bl:22, Bl:24, Bl:25, Bl:26, Ca-VII, Ca-VIII, NL341, NL519.
Another undesired feature of existing green lettuce plants that have a large number of small, more uniform leaves (U.S. Pat. No. 6,320,104; US patent application 20070022496), is that they show multiple apices in warm growing conditions with sufficient moisture available. In these conditions the apex is splitting up in several independent apices. An example of such warm growing conditions can be a daytime temperature of 30° C. and a night time temperature of 20° C. at a day length of 14 hours. If these multiple apices appear in an early growing stage it makes the plant unsuitable for marketing as wholehead, and much less suitable for harvesting single leaves. If these multiple apices appear in a later growing stage, it makes the plants less marketable as wholehead. Those plants can still be used for single leaf harvesting.
A need exists, therefore, for an improved green lettuce variety, which exhibits resistance to downy mildew and exhibits abundant leaf growth without multiple apices.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.